A facility for continuously manufacturing a hot-dip galvanized steel sheet conventionally includes galvanization performed by continuously annealing a steel strip, which is a strip-shaped steel sheet, using a continuous annealing furnace and by subsequently feeding the annealed steel strip into a plating bath of zinc or zinc alloy directly through a snout on the exit side of the continuous annealing furnace. The continuous annealing furnace commonly includes a heating zone in which a steel sheet that is being transported through an airtight furnace is heated to a temperature of about 800° C. to 1000° C. in a high-temperature gas atmosphere and a cooling zone in which the heated steel sheet is cooled to a temperature of about 300° C. to 600° C. by spraying a low-temperature gas. There is also a continuous annealing furnace having a soaking zone, in which the heated steel strip is soaked, subsequent to the heating zone. There is also a continuous annealing furnace having a preheating zone, in which the steel strip which has not been heated is preheated, prior to the heating zone.
Patent Literature 1 describes a bright annealing furnace, which is a facility having only a furnace without a snout, provided with a ventilation pipe which is placed at the boundary of an interior refractory and an exterior steel shell of the furnace wall. The gas inside the furnace is discharged to the outside of the furnace through the ventilation pipe, and a furnace gas circulation device, into which the discharged gas is suctioned in order to clean the gas by removing impurities in the gas and from which the cleaned gas is returned into the furnace, is additionally installed outside the furnace. Thereby, in particular, a seasoning time, or the time required to start up a new furnace at the beginning of its operation or to start up a repaired furnace when the furnace is resumed to operate, is significantly decreased.
Patent Literature 2 describes a technique using an apparatus for continuous annealing in a reducing atmosphere, which is an apparatus having only a furnace without a snout, for a metal strip. A refiner, which is a moisture removing device and is referred to as a refining device in Patent Literature 2, is utilized. Thereby, the cooling efficiency is increased by spraying the gas in a preheating zone, which provided prior to a heating zone, into a cooling zone, or the preheating efficiency is increased by conversely spraying the gas in the cooling zone into the preheating zone.